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Indonesian Droughts and Their Teleconnections

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Indonesian Droughts and Their Teleconnections Berliner Geographische Studien 1986 Band 20 Seite 251 - 265 INDONESIAN DROUGHTS AND THEIR TELECONNECTIONS von Hermann Flohn, Bonn ZUSAMMENFASSUNG: Indonesische Dürren und ihre Fern-Zusammen- hänge El-Nino-Southern Oscillation (ENSO)-Ereignisse an der Westküste Südamerikas und auf dem äquatorialen Pazifik werden begleitet von gleichzeitigen Dürreperioden in Indonesien. Während der Monate Juni-November 1982 litten große Gebiete von Indonesien (vor allem südlich des Äquators) unter einer schweren Dürre, in der auf einem Gebiet von mehreren Mio. km2 bis herab zu 10% (und weniger) der normalen Regenmenge fielen; in dem sonst extrem feuchten Urwaldgebiet von Ost- Kalimantan kam es zu riesigen Waldbränden. Aufgrund von kohärenten Reihen von Gebietsmitteln der Regenmenge für Süd-Indonesien (1879- 1970) und Papua-Niugini (1904-1972) werden die zeitlichen Schwankungen der Dürrehäufigkeit und ihre Fern-Zusammenhänge mit entsprechenden Niederschlagsreihen für die Line Islands, Nauru und Sri Lanka untersucht. Sie belegen ebenso auch die Gleichzeitigkeit von besonders hefti- gen Niederschlägen in Indonesien mit Dürren (und wahrscheinlich aufquellendem Kaltwasser) auf dem Zentral-Pazifik, die mindestens bis Nauru (169°E) reichen. Lokale Änderungen der Wassertem- peratur im Südchinesischen Meer (NE Singapore) müssen als sekundäre Konsequenzen aufgefaßt werden. ABSTRACT El-Nino-Southern Oscillation (ENSO) events at the west coast of South America and the equatorial Pacific are accompanied by drought periods in the "maritime continent" Indonesia (BEHREND, 1984). During June to November 1982, many areas of Indonesia (mainly south of the equator) suffered from a loss of up to 90 and more percent of normal rainfall over an area of several million km2, causing extended wildfires in (otherwise excessively humid) eastern Kalimantan. Coherent area-averaged rainfall series for southern Indonesia (1879-1970) and Papua-Niugini (1904-1972) are used to investigate the time fluctuation of droughts and their teleconnection with similar long rainfall series at the Line Islands, at Nauru and Sri Lanka. They indicate also a coincidence between heavy rainfall at Indonesia with droughts (and probably upwelling) extending across the Central Pacific at least to Nauru (Long. 169°E). Local changes of the sea-surface temperature in the South China Sea NE of Singapore appear to be a secondary consequence of droughts. INTRODUCTION After two remarkable El Nino events related to global-scale climatic anomalies (1972 and 1982/83) the interest of elimatologists has been concentrated on these 251 world-wide anomalies, which have been reviewed by several authors (RAS- MÜSSEN & CARPENTER, 1982; RASMUSSEN & WALLACE, 1983; ARKTN, 1984; WRIGHT, 1985; LATIF, 1986). The anomalies apparently spread from the central tropical Pacific to the South. American coast, where they are responsible for the warm water events known since a long time ago äs El Nino (SCHOTT, 1931; DOBERTTZ, 1968). They are strongly related to a large_-scale see-saw correlation between surface pressures at the tadian Ocean (including Indonesia) and at the Pacific, detected äs early äs around 1924 by Sir Gilbert Walker (see BERLAGE, 1957; BJERKNES, 1969) and described äs Southern Oscillation. Nowadays the strong coupling of oceanic and atmospheric phenomena in an area covering more than half of the earth's circumference is often shortly called ENSO (El Nino- Southern Oscillation). One of the most remarkable phenomena of these ENSO events are large-scale droughts in Indonesia, which shall be discussed here in greater detail, after BERLAGE (1957), QUTNN et al. (1978); NICHOLLS (1981; 1984) and BEHREND (1984) have shown the coincidence between them and the Pacific ENSO events. In bis diploma thesis, BEHREND has evaluated an area-averaged precipitation record .for southern Indonesia (13 statistically coherent stations between Lat. 2°S and 12°S and Long. 106-130°E, for the period 1879-1970, Fig. 1). This will be used here, together with Wright's SO-Index (WRIGHT, 1975). In Chapter 2 the excessive drought of 1982/83 and its extension shall be described, äs fax äs the incomplete data sources allow. Then the mechanism of convective clouds and rainfall in Indonesia in its relation to the temperature distribution over this "maritime continent" (RAMAGE, 1968) shall be discussed. Chapter 4 deals with the history, frequency and intensity of droughts between 1879 and 1970. In the next chapter the unusually high sea surface temperature and its evolution during this Century shall be described. In chapter 6 we shall discuss some of the rainfall teleconnections found by BEHREND in the equatorial belt running from Indonesia across the Indian Ocean and the Pacific, even into the Caribbean, thus extending over more than 250° of Longitude. 2 THE 1982/83 DROUGHT The drought of 19_82/83 was certainly not the first of this kind (see chapter 4), but it was the first routinely documented from satellites by measuring cloud motions (and thus the wind distribution in two levels) and the energy of the outgoing longwave radiation of the cloud surface. Using the Stefan-Boltzmann Law, together with the longwave emission coefficient of clouds, these data - received "on line" at the Climatic Diagnostic Center'of the United States near Washington, D.C. - allow a reliable estimate of cloud top height and temperature, which are strongly correlated with intensity and amount of convective rainfall. While normally three large areas of cloud top temperature below 210 K (or even below 200 K) are observed above Amazonia, equatorial Africa and (the largest and most intensive of all) above Indo- nesia, during this drought the last one gradually moved eastward across the date line to Long. 140-150°W, i.e. over a distance of more than 8 000 km. Unfortunately, monthly surface data to describe this droiaght are incomplete; for Indonesia they are only available from June 1982 onwards, with many Station changes and_ gaps from month to month. For neighbouring areas (i.e. Malaysia, Papua-Niugini, Australia, Pacific Islands, Philippines) the Situation is slightly better, but by no means ideal; in all cases the density of reporting stations is far too low. As an example, Borneo (Kalimantan) with an area of 0.75 x 106 km.2 is represented by no more than six stations, of which only two (in Malaysia) are complete. 252 Fig. 1: Approximate rainfall anomalies(%) in Indonesia during June-November 1982. Dots = stations; circles = 13 stations of the 1879-1970 series for southern Indonesia (BEHREND, 1984). 253 In spite of deficiences I have tried to put these data together and to draw an approximative outline (Fig. 1) which is based on some 40 Station values for the half- year June-November (6-11) 1982, the majority of them imcomplete. Here this ex- periment can be justified by the high serial (auto-) correlation of the Indonesian rainfall records during this season, which amounts (BEHREND, 1984) up to 0.79. Surprisingly enough, this is about äs high äs at Station records along the Pacific equator, there due to the persistent large anomalies of sea surface temperature (SST). Consequently, the assumption that missing monthly values show anomalies of the same sign (and of a similar amount) äs the observed data, is well justified. Fig. l shows rainfall amount during June-November 1982 in per cent of long-term averages. An area with less than 10% reaches from Tanimbar Islands (southeast of Ceram) to central Sulawesi and eastern Java. The isoline 25% includes northern Sulawesi, Halmahera and westernmost New Guinea (Irian) with an area of more than 2 x 106 km.2. Eastern Kalimantan and the greater part of New Guinea are probably situated in an area with less than 50% of the normal. Normally the dry season controls the area south of about Lat. 7°S; however, great fluctuations from year to year are by no means uncommon (RAMAGE, 1968, see chapter 5). At least in Kalimantan, the intensity of the drought is not adequately represented by these rainfall data. In its eastern part, visitors were surprised by the quite unusual dryness of the tropical rain forest: dry leaves rustling underfoot äs in midlatitude deciduous forests during autumn. In the last months of 1982 up to 35 000 km2 of (partly yirgin) forest was destroyed by wildfires (FRANKF. ALLG. ZEIT. 12.9.1984) - a Situation unheard of in an area with an annual rainfall of 3-5 000 mm. Even more surprising was a newspaper report (however not yet con- firmed) that the Fly river feil temporarily dry - a river draining the central high- lands of New Guinea with one of the highest specific discharges of all rivers: 41 l/s per km.2. During June-November Menado (NE Sulawesi) received 94 instead of 1132 mm (long-term average), Den Pasar (Bali) 14 instead of 500 mm, Singapore 531 instead of 1603 mm. In "Monthly Climatic Data for the World", data from (in the average) 37 stations from Indonesia were published, representing an area near 8 x 106 km.2: only 9% of these rainfall data were larger than normal. Unfortunately no data for May 1982 or before were available. In parts of Indonesia the drought lasted until April or even May 1983. As an example, Kota Kinabalu (Sabah) and Ujung Pandang (Sulawesi) received, during the 11 months June 1982 to April 1983 only 48 resp. 42% of normal (which amounts to about 2 500 mm), while Gorontalo (Sulawesi) and Ambon (only 10 month) re- ceived 33 resp. 31%, and Den Pasar (in 9 month) 22%. The 1982 drought extended to the greater part of the Australian continent: from a sample of 25 stations east of Long. 129°E, 87% of all monthly rainfall data were below normal during the months April-December 1982, in the western part (11 stations) 74%. Similarly the southern and central Philippines suffered from the drought. After the turn of the year, the main drought region was displaced to the western Pacific: from the 7 islands of Micronesia (7-13° N, 135-173°E) three received only 11-12%, the remaining four 23- 35% of the normal.
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